ATLANTIC REGION 575 



graptus conferta, and Diplograptus denlatus. The first of this series is 

 diagnostic of our Middle and the last of our Upper Deepkill zone, the 

 Deepkill on the whole being thus equivalent to the Arenig. 



While in the Hudson A^alley the Upper Deepkill beds appear to be 

 succeeded disconformably by the Normanskill beds of Upper Chazy or 

 Black Eiver age, the Arenig of Wales seems to be conformably succeeded 

 by the Llandeilo. No positive evidence is at present available to show 

 whether there is or is not a hiatus between these two formations in 

 Britain, but I am inclined to think that careful search will reveal its 

 existence in some sections. In South Wales the Llandeilo has a thick- 

 ness of 2,000 feet and is divisible into the following members : 



Feet 



Upper Llandeilo slates 1,000 



Llandeilo limestone 200 



Lower Llandeilo slates 800 



The lowest zone of the Llandeilan is characterized by Didy mo graptus 

 murchisoni, the Middle by Diplo graptus foliaceus and Glimaco graptus 

 scharenbergi, and the Upper by Crypto graptus tricornis, Glimaco graptus 

 scharenbergi, Coenograptus {N emagraptus) gracilis, and Dicello graptus 

 sextans. This association of species in the Upper Llandeilo is also char- 

 acteristic of the Normanskill beds of New York, which thus appear to 

 represent the exact equivalent of the Upper Llandeilo. Characteristic 

 Lower Llandeilo trilobites are Asaplius tyrannus, Calymene cambrensis, 

 Trinucleus lloydii, and T. favus, while those of the Upper Llandeilo in- 

 clude Barrandia cordai, Cheirurus sedgwicki, and Ogygia buchii. 



Scotland. — In the southern uplands of Scotland both Arenig and Llan- 

 deilo rocks are present. The former are represented by radiolarian cherts 

 and by mudstones of unknown thickness. They appear to overlap the 

 Tremadoc and probably represent only a part of the Arenig series. In 

 Dumfriesshire they are complicated by volcanic flows, which also overlie 

 and separate them from the Llandeilo. There seems to be a disconformity 

 in this region between the Arenig and the overlying Glenkiln shales, 

 which carry a Normanskill or Upper Llandeilo fauna. 



The disconformity and hiatus is well marked in the Girvan district of 

 southwest Scotland (Ayrshire), where the Lower Llandeilo is overlapped 

 by the Upper, which rests with a basal conglomerate on the Arenig cherts. 



The Arenig or Ballantrae series contain : 12 



Phyllograptus typus Hall 

 Tetragraptus quadribrachiatus Hall 



12 Charles Lapworth : On the Ballantrae rocks of south Scotland and their place in 

 the Upland Sequence. Geological Magazine, n. s., Dec. 3, vol. vi, 1889, pp. 20-27 (22). 



